The invention lies in the field of catalytic converters. The invention relates to a device for the catalytic removal of a pollutant from the exhaust gas of a combustion system, such as a fossil-fired power plant or an internal combustion engine. The combustion system has an exhaust-gas duct for carrying the exhaust gas and an injection device for introducing a reagent into the exhaust gas in an injection space of the exhaust-gas duct. A catalytic converter is installed in the exhaust-gas duct. The injection space and a mixing space are configured as a muffler. The mixing space is connected downstream if appropriate and is provided for the purpose of mixing the reagent with the exhaust gas.
A catalytic removal device is disclosed, for example, in European Patent Application 0 779 415 A1.
When a fossil fuel is burnt in a combustion system, considerable quantities of pollutants, such as nitrogen oxides (NOx), hydrocarbons, carbon monoxide, and oxides of sulfur, are formed. If the combustion temperatures are high enough, dioxins and furans are also formed. These pollutants can enter the environment through the exhaust gas from the combustion system. A combustion system of this kind can include a boiler system or a fossil-fired power plant, for example, or even an internal combustion engine.
Due to strict legal requirements that limit emissions of the above mentioned pollutants, it is necessary to treat the exhaust gas from combustion systems for reducing the quantity of pollutants it contains. Many different catalytic converters have been developed for such a purpose. Thus, for example, the prior art includes DeNOx catalysts for reducing the quantity of nitrogen oxides. Such catalysts convert the nitrogen oxides (NOx) contained in the exhaust gas by a reducing agent, generally ammonia or urea, into environmentally neutral nitrogen and water by the process known as selective catalytic reduction (SCR). The reducing agent is introduced into the exhaust gas ahead of the catalytic converter in the direction of flow of the exhaust gas. The reducing agent then enters the catalytic converter, as far as possible as a homogeneous mixture with the nitrogen oxides contained in the exhaust gas. It is also possible in this context for a number of catalytic converters to be disposed in series.
For reducing the quantity of pollutants in the exhaust gas of an Otto engine, the prior art includes catalysts containing noble metals, for example, on which hydrocarbons and carbon monoxide together with nitrogen oxides are converted into carbon dioxide, nitrogen, and/or water.
The above mentioned DeNOx catalyst can be used to remove nitrogen oxides from the exhaust gas of a diesel engine. Here, urea is preferably used. The urea is injected into the flow of exhaust gas in a manner appropriate to the application. With such an electronically controlled diesel catalytic converter, it is possible to reduce considerably the level of nitrogen oxides in the exhaust gas of the diesel engine.
An SCR device for the catalytic removal of NOx in the exhaust gas of a combustion system is first of all required to effect a significant and effective reduction in NOx emissions. It is also generally required to reduce sound emissions. The sound results from the operation of the combustion system and represents a problem for the environment. At the same time, there is a demand for minimizing the space required. To reduce sound emissions, provision has hitherto been made for the installation of a separate muffler in the exhaust-gas duct. The separate muffler made it possible to reduce sound emissions to below the permissible limits. However, there are noticeable disadvantages with the muffler solution, to wit, additional costs, an increased pressure drop, and a larger space requirement.
It is accordingly an object of the invention to provide a device for catalytic exhaust-gas purification that overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type and that requires no unnecessary space for a separate muffler and further reduces sound emissions.
The invention is based on the consideration that the objective can be achieved if different functions can be combined in an already existing component.
With the foregoing and other objects in view, there is provided, in accordance with the invention, a device for catalytic removal of a pollutant from an exhaust gas of a combustion system, including an exhaust-gas duct for carrying exhaust gas, a reagent, a muffler having an injection space and a mixing space for mixing the reagent with the exhaust gas, an injection device for introducing the reagent into the exhaust gas in the injection space, at least one catalytic converter disposed in the exhaust-gas duct, and mixers disposed at least one of before and after the injection space with respect to a direction of flow of the exhaust gas, the mixers defining a distance therebetween, the distance matched to a frequency of sound to be damped in the exhaust gas. Particularly, the combustion system is a fossil-fired power plant or an internal combustion engine.
An advantage of such solution is that it is possible to construct an SCR catalytic converter system for the exhaust gas of a combustion system in a very compact and economical manner because the injection and mixing space are configured as a muffler. For noise damping, the injection space and/or the mixing space can then be lined with a perforated plate or a thin, flexible single- or double-layer plate. The injection space and/or the mixing space is/are preferably provided with a sound-deadening lining.
In accordance with another feature of the invention, the mixing space is disposed downstream of the injection space.
In accordance with an added feature of the invention, the perforated plate, the flexible single-layer plate, and/or the flexible double-layer plate are backed with a sound-absorbing material selected from the group consisting of steel wool, ceramic wool, steel web, and fiber web.
In accordance with an additional feature of the invention, the exhaust gas duct has an inlet part and a 180xc2x0 deflection for the exhaust gas, and the 180xc2x0 deflection is disposed between the inlet part and the at least one catalytic converter, and including at least one of a reflection damper and a absorption damper disposed in a region of the 180xc2x0 deflection.
In accordance with yet another feature of the invention, the injection space and the mixing space form a constructional or structural unit.
In accordance with yet a further feature of the invention, the structural unit has an inflow side and an outflow side and at least one of the inflow side and the outflow side is a pipe resonator.
In accordance with yet an added feature of the invention, the 180xc2x0 deflection is disposed on the outflow side of the structural unit.
In accordance with a concomitant feature of the invention, the injection device has a nozzle and a jacketing tube in a region at the nozzle, and the jacketing tube is aligned in a direction of flow of the exhaust gas.
Other features that are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a device for catalytic exhaust-gas purification, it is, nevertheless, not intended to be limited to the details shown because various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.